1. Field of the Invention
The present invention relates to a color thermal printer of a thermal wax transfer type, and more particularly to a color thermal printer suitable for printing both black character hard copies and color half-tone image hard copies.
2. Description of Related Background Art
Recently, thermal printers of a thermal wax transfer type are widely used. This type of printer has a thermal head having a number of heating elements linearly disposed, and the thermal head is made in contact with the back surface of an ink film to transfer softened or melted ink to a recording sheet. Such a thermal printer includes a black-and-white thermal printer for printing black characters and line images by using a black ink film, and a color thermal printer for printing color half-tone images and color characters by using a color ink film.
A color thermal printer includes a printer with a single thermal head and a printer with a plurality of thermal heads. A color thermal printer with a single thermal head, such as disclosed in Japanese Patent Publication No. 63-65030, uses a color ink film formed thereon with cyan, magenta, yellow, and black ink areas at a constant pitch. In this case, color images and characters are recorded on a recording sheet by passing the sheet four times on the thermal head and sequentially recording four colors on the same frame. A color thermal printer with a plurality of thermal heads has a cyan thermal head for heating a cyan ink film, a magenta thermal head for heating a magenta ink film, a yellow thermal head for heating a yellow ink film, and a black thermal head for heating a black ink film. Each thermal head is provided with such a monochromatic ink film. Color images and color characters are recorded on a recording sheet at the same time for all four colors by passing the sheet only once.
A thermal printer used with a personal computer for example is used mostly for printing characters and line images by using black ink, and relatively less for printing color half-tone images. When a color printer with a single thermal head is used, black characters are printed using a black ink area while not using the other cyan, magenta and yellow ink areas. The latter ink areas are fed without contributing to printing, resulting in uneconomical use. When a color printer with four thermal heads is used, there is no wasteful area of color ink films for both black characters and color half-tone images, but the size and manufacturing cost of the printer become large because of the use of the four thermal heads. Furthermore, the width of a heating element of a conventional thermal head is comparable to the length of a virtual picture cell on a recording sheet in the sub scan direction, and each ink dot is printed on the whole area of a picture cell, so that a half-tone color, image cannot be printed without using matrix half-tone printing.
A thermal wax transfer printing method capable of printing a half-tone image has been proposed in U.S. Ser. No. 7/699.591 filed on May 14, 1991. According to this thermal wax transfer printing method, a thermal head is intermittently moved in the unit feeding length relative to a recording sheet, the unit feeding length being set shorter than the width (or length in the sub scan direction) of each heating element. Each heating element is driven on the unit feeding length to change the record length within each picture cell in the sub scan direction, in accordance with the recording density. However, the thermal head used with this thermal wax transfer printing method has a heating element whose width (or length in the sub scan direction) is narrow, leaving a problem of short lifetime and low printing speed. In practical use, printing of a half-tone color image is not frequent, but printing of characters is very often. Therefore, there arises a problem of low printing speed when characters are printed frequently.